1. Field of the Invention
The present invention relates to electronic circuits and, more specifically, to the control of a switch of a relatively high A.C. voltage by means of a D.C. signal of relatively low amplitude, through an isolation barrier.
An example of application of the present invention relates to control circuits used in electric household appliances (for example, washing machines, ovens, refrigerators, etc.). More generally, the present invention applies to any electric appliance to be controlled from a signal of low-voltage with respect to a high A.C. voltage which supplies it.
2. Discussion of the Related Art
The control of a high-voltage load powered by a high A.C. voltage (for example, the mains voltage of several hundreds of volts) by means of a low-voltage D.C. electronic circuit (at most a few tens of volts) requires an isolation barrier between this electronic circuit and the A.C. switch. Such an isolation barrier is used not only to protect the low-voltage circuit but also to protect the user against possible electrocution when he actuates control elements linked to the electronic circuit. Further, A.C. switches are integrated elements that are bidirectional for both current and voltage, with a control requiring a low-voltage D.C. voltage to restart the switch on each halfwave of the A.C. voltage. These are, for example, triacs or other integrated devices performing the same function. A D.C.-to-D.C. converter with an isolation barrier is then necessary to perform such functions.
Isolation transformers having their primary winding excited by a high-frequency signal (D.C. or A.C. pulse signal) rectified and filtered at the secondary winding are generally used to turn on the A.C. switch. For example, document WO-A-2006/023767 describes a converter using a transformer in an H bridge to generate a power supply isolated from the primary winding. The use of an isolation transformer is bulky and expensive.
Other systems (for example, such as that described in U.S. Pat. No. 6,728,320) use capacitive elements to transmit a control signal between an integrated circuit and an A.C. switch control circuit. Such a transmission, however, only concerns the control set point and requires, on the A.C. switch side, generation of a low supply voltage and thus another transformer to provide the power necessary for the successive switch triggerings.
Optocouplers may also be used, but this poses reliability problems without avoiding the need for a power supply at the secondary.